This invention relates in general to the field of chromatography and in particular to an improved method and apparatus for use in the field of liquid chromatography.
Chromatography is recognized by those skilled in the art as a procedure for analyzing a sample through the steps of passing the sample through a body of material and detecting the relative separations of various sample substances which occur during such passage. In the specific field of liquid chromatography, the sample undergoing analysis may typically be introduced into a carrier liquid which is passed through a chromatography column containing material which adsorbs components of the liquid being analyzed. The extent to which these different components are adsorbed in the column is determined by a suitable detector connected to analyze the liquid effluent from the column.
Those familiar with the art of liquid chromatography are aware that it is highly desirable to provide a flow of carrier liquid which is delivered to the chromatograph column at a constant rate of flow and which is free of any appreciable pressure pulsations. The presence of pressure pulsations in the carrier liquid supplied to the chromatograph column can vary the rates of adsorption occurring within the column, and thus can provide a false indication of the adsorption of constituents in the sample being analyzed. These false indications amount to "noise" in the detector output, and this noise masks or otherwise obscures bona fide signals whose amplitude is not significantly greater than that of the pulsation-induced noise. The effective sensitivity of a liquid chromatograph can be increased, accordingly, by reducing the pressure pulsations in the carrier liquid. Various pumping means may be employed to deliver high pressure carrier liquid at constant flow rates with minimum pressure pulsations, and one such liquid chromatography system employing substantially constant flow and pressure pumping means is disclosed in U.S. Pat. No. 3,932,067, issued Jan. 13, 1976.
In liquid chromatographic analysis, it is a practice to first establish a flow of high pressure carrier liquid through the chromatography column and then to introduce a precisely measured volume of sample liquid into the flow stream so that it is carried to the column for separation and subsequent analysis. One approach to sample introduction involves the use of a syringe injection method wherein a syringe containing a precise amount of sample liquid is injected through an elastomeric septum directly into the carrier flow path. However, direct sampling injection has a disadvantage, in that, the elastomeric septum is often incompatible with many carrier liquids. Also, because of the high operating pressures of the carrier liquid, e.g., pressures as much as 1,000 psi, special syringes with high pressure capabilities must be employed.
Another approach to sample introduction involves the use of sample injection valve having a liquid-holding sample loop, or conduit, of fixed volumetric capacity, which is normally external of the carrier liquid conduit and which may be charged with a desired predetermined volume of sample fluid. The valve is then actuated to connect the external sample loop into the carrier liquid circuit and the sample previously disposed in the loop is carried to the separation column. One such sample injection valve mechanism utilizing an external sample loop to introduce a predetermined measured amount of sample liquid into a diverted portion of a pressurized carrier liquid stream is disclosed in U.S. Pat. No. 3,916,692.
Since it is necessary for accurate sample analysis that the high pressure liquid through the column be free of any appreciable flow and pressure variations, it is highly desirable that the liquid sample be introduced into the carrier liquid without creating any pressure or flow variations which would disrupt the analysis of the sample. It can be understood that when a small sample of specimen of liquid is momentarily introduced from an external storage source by valving means into the high pressure carrier liquid, a large pressure variation can occur in the liquid stream as a result of the valve-switching operation with detrimental effects to the analysis obtained by the apparatus. This is particularly true when the storage source is filled and maintained at substantially atmospheric pressure before its introduction into the high pressure carrier flow stream.
Due to the high pressures employed in the carrier system, it is also desirable to insure that the sample injection valve mechanism for introducing the sample liquid is maintained free from leakage both during the accumulation of a precise amount of the sample, as well as during introduction of the sample into the carrier liquid stream. Furthermore, to insure that precise amounts of liquid sample are delivered to the chromatography column, it is essential that the sample injection valve mechanism be such as to minimize the dead space in which liquid sample is retained upon injection of the liquid sample into the carrier liquid stream.